If this particular section of code isn't already familiar to you, you
probably need to take a step back and first review the Forms article
before proceeding.

Assume for a moment that this form utilizes an imaginary "Product" class
that has only two properties ("name" and "price"). The form generated from
this class will look the exact same regardless if a new Product is being created
or if an existing product is being edited (e.g. a product fetched from the database).

Suppose now, that you don't want the user to be able to change the name value
once the object has been created. To do this, you can rely on Symfony's
EventDispatcher component
system to analyze the data on the object and modify the form based on the
Product object's data. In this article, you'll learn how to add this level of
flexibility to your forms.

So, instead of directly adding that name widget, the responsibility of
creating that particular field is delegated to an event listener:

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// src/Form/Type/ProductType.phpnamespaceApp\Form\Type;// ...useSymfony\Component\Form\FormEvent;useSymfony\Component\Form\FormEvents;classProductTypeextendsAbstractType{publicfunctionbuildForm(FormBuilderInterface$builder,array$options){$builder->add('price');$builder->addEventListener(FormEvents::PRE_SET_DATA,function(FormEvent$event){// ... adding the name field if needed});}// ...}

The goal is to create a name field only if the underlying Product
object is new (e.g. hasn't been persisted to the database). Based on that,
the event listener might look like the following:

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// ...publicfunctionbuildForm(FormBuilderInterface$builder,array$options){// ...$builder->addEventListener(FormEvents::PRE_SET_DATA,function(FormEvent$event){$product=$event->getData();$form=$event->getForm();// checks if the Product object is "new"// If no data is passed to the form, the data is "null".// This should be considered a new "Product"if(!$product||null===$product->getId()){$form->add('name',TextType::class);}});}

Note

The FormEvents::PRE_SET_DATA line actually resolves to the string
form.pre_set_data. FormEvents
serves an organizational purpose. It is a centralized location in which
you can find all of the various form events available. You can view the
full list of form events via the
FormEvents class.

// src/Form/EventListener/AddNameFieldSubscriber.phpnamespaceApp\Form\EventListener;useSymfony\Component\EventDispatcher\EventSubscriberInterface;useSymfony\Component\Form\Extension\Core\Type\TextType;useSymfony\Component\Form\FormEvent;useSymfony\Component\Form\FormEvents;classAddNameFieldSubscriberimplementsEventSubscriberInterface{publicstaticfunctiongetSubscribedEvents(){// Tells the dispatcher that you want to listen on the form.pre_set_data// event and that the preSetData method should be called.return[FormEvents::PRE_SET_DATA=>'preSetData'];}publicfunctionpreSetData(FormEvent$event){$product=$event->getData();$form=$event->getForm();if(!$product||null===$product->getId()){$form->add('name',TextType::class);}}}

Sometimes you want a form to be generated dynamically based not only on data
from the form but also on something else - like some data from the current user.
Suppose you have a social website where a user can only message people marked
as friends on the website. In this case, a "choice list" of whom to message
should only contain users that are the current user's friends.

// src/Form/Type/FriendMessageFormType.phpnamespaceApp\Form\Type;useSymfony\Component\Form\AbstractType;useSymfony\Component\Form\Extension\Core\Type\TextareaType;useSymfony\Component\Form\Extension\Core\Type\TextType;useSymfony\Component\Form\FormBuilderInterface;useSymfony\Component\Form\FormEvent;useSymfony\Component\Form\FormEvents;classFriendMessageFormTypeextendsAbstractType{publicfunctionbuildForm(FormBuilderInterface$builder,array$options){$builder->add('subject',TextType::class)->add('body',TextareaType::class);$builder->addEventListener(FormEvents::PRE_SET_DATA,function(FormEvent$event){// ... add a choice list of friends of the current application user});}}

The problem is now to get the current user and create a choice field that
contains only this user's friends. This can be done injecting the Security
service into the form type so you can get the current user object:

// src/Form/Type/FriendMessageFormType.phpuseApp\Entity\User;useDoctrine\ORM\EntityRepository;useSymfony\Bridge\Doctrine\Form\Type\EntityType;useSymfony\Component\Form\Extension\Core\Type\TextareaType;useSymfony\Component\Form\Extension\Core\Type\TextType;useSymfony\Component\Security\Core\Security;// ...classFriendMessageFormTypeextendsAbstractType{private$security;publicfunction__construct(Security$security){$this->security=$security;}publicfunctionbuildForm(FormBuilderInterface$builder,array$options){$builder->add('subject',TextType::class)->add('body',TextareaType::class);// grab the user, do a quick sanity check that one exists$user=$this->security->getUser();if(!$user){thrownew\LogicException('The FriendMessageFormType cannot be used without an authenticated user!');}$builder->addEventListener(FormEvents::PRE_SET_DATA,function(FormEvent$event)use($user){if(null!==$event->getData()->getFriend()){// we don't need to add the friend field because// the message will be addressed to a fixed friendreturn;}$form=$event->getForm();$formOptions=['class'=>User::class,'choice_label'=>'fullName','query_builder'=>function(UserRepository$userRepository)use($user){// call a method on your repository that returns the query builder// return $userRepository->createFriendsQueryBuilder($user);},];// create the field, this is similar the $builder->add()// field name, field type, field options$form->add('friend',EntityType::class,$formOptions);});}// ...}

Note

You might wonder, now that you have access to the User object, why not
just use it directly in buildForm() and omit the event listener? This is
because doing so in the buildForm() method would result in the whole
form type being modified and not just this one form instance. This may not
usually be a problem, but technically a single form type could be used on a
single request to create many forms or fields.

Another case that can appear is that you want to customize the form specific to
the data that was submitted by the user. For example, imagine you have a registration
form for sports gatherings. Some events will allow you to specify your preferred
position on the field. This would be a choice field for example. However the
possible choices will depend on each sport. Football will have attack, defense,
goalkeeper etc... Baseball will have a pitcher but will not have a goalkeeper. You
will need the correct options in order for validation to pass.

The meetup is passed as an entity field to the form. So we can access each
sport like this:

When you're building this form to display to the user for the first time,
then this example works perfectly.

However, things get more difficult when you handle the form submission. This
is because the PRE_SET_DATA event tells us the data that you're starting
with (e.g. an empty SportMeetup object), not the submitted data.

On a form, we can usually listen to the following events:

PRE_SET_DATA

POST_SET_DATA

PRE_SUBMIT

SUBMIT

POST_SUBMIT

The key is to add a POST_SUBMIT listener to the field that your new field
depends on. If you add a POST_SUBMIT listener to a form child (e.g. sport),
and add new children to the parent form, the Form component will detect the
new field automatically and map it to the submitted client data.

// src/Form/Type/SportMeetupType.phpnamespaceApp\Form\Type;useApp\Entity\Sport;useSymfony\Bridge\Doctrine\Form\Type\EntityType;useSymfony\Component\Form\FormInterface;// ...classSportMeetupTypeextendsAbstractType{publicfunctionbuildForm(FormBuilderInterface$builder,array$options){$builder->add('sport',EntityType::class,['class'=>'App\Entity\Sport','placeholder'=>'',]);;$formModifier=function(FormInterface$form,Sport$sport=null){$positions=null===$sport?[]:$sport->getAvailablePositions();$form->add('position',EntityType::class,['class'=>'App\Entity\Position','placeholder'=>'','choices'=>$positions,]);};$builder->addEventListener(FormEvents::PRE_SET_DATA,function(FormEvent$event)use($formModifier){// this would be your entity, i.e. SportMeetup$data=$event->getData();$formModifier($event->getForm(),$data->getSport());});$builder->get('sport')->addEventListener(FormEvents::POST_SUBMIT,function(FormEvent$event)use($formModifier){// It's important here to fetch $event->getForm()->getData(), as// $event->getData() will get you the client data (that is, the ID)$sport=$event->getForm()->getData();// since we've added the listener to the child, we'll have to pass on// the parent to the callback functions!$formModifier($event->getForm()->getParent(),$sport);});}// ...}

You can see that you need to listen on these two events and have different
callbacks only because in two different scenarios, the data that you can use is
available in different events. Other than that, the listeners always perform
exactly the same things on a given form.

Tip

The FormEvents::POST_SUBMIT event does not allow to modify the form
the listener is bound to, but it allows to modify its parent.

One piece that is still missing is the client-side updating of your form after
the sport is selected. This should be handled by making an AJAX call back to
your application. Assume that you have a sport meetup creation controller:

{# templates/meetup/create.html.twig #}{{form_start(form)}}{{form_row(form.sport)}}{# <select id="meetup_sport" ... #}{{form_row(form.position)}}{# <select id="meetup_position" ... #}{# ... #}{{form_end(form)}}<script>var$sport=$('#meetup_sport');// When sport gets selected ...$sport.change(function(){// ... retrieve the corresponding form.var$form=$(this).closest('form');// Simulate form data, but only include the selected sport value.vardata={};data[$sport.attr('name')]=$sport.val();// Submit data via AJAX to the form's action path.$.ajax({url:$form.attr('action'),type:$form.attr('method'),data:data,success:function(html){// Replace current position field ...$('#meetup_position').replaceWith(// ... with the returned one from the AJAX response.$(html).find('#meetup_position'));// Position field now displays the appropriate positions.}});});</script>

The major benefit of submitting the whole form to just extract the updated
position field is that no additional server-side code is needed; all the
code from above to generate the submitted form can be reused.